Polyisoprene is a naturally occurring unsaturated hydrocarbon polymer isolated from the latex of rubber trees, the primary source of natural rubber. Polyisoprene biosynthesis can selectively yield two geometrical isomers of polyisoprene, cis-1,4-polyisoprene (from Hevea brasiliensis) and trans-1,4-polyisoprene (from Gutta percha). Over ten million tons of the natural elastomers are harvested annually to manufacture materials such as tires and gloves. Rubber tree plantations are restricted to tropical climate, e.g. Southeast Asia and West Africa, where extensive farming supplants food crops and contaminates soils due to heavy use of arsenic-based pesticides, with severe geopolitical consequences.
About 15 million tons of synthetic rubber are produced annually. Because chemists have not yet found a satisfactory and practical way to produce rubber with appropriate control of the double bond geometry, natural rubber is still the preferred material when high quality rubber is needed, such as in airplane tires. Natural rubber does not become brittle even at low temperatures due to a high 1,4-microstructure content which allows for a low glass transition temperature (less than −65° C.). A high 1,4-microstructure content, such as pure 1,4-cis or pure trans, also allows natural rubber to undergo strain crystallization, providing an elastomeric material with enhanced mechanical properties. Generation of a polymer with a high cis-1,4 or trans-1,4 microstructure content is one way of obtaining elastomers with optimal properties. On the other hand, a reasonable 3,4-microstructure content (from 5-15%) can allow for further chemical modification of the synthetic rubber produced as an alternative to natural rubber. Moreover, the side-chain olefin can be modified post-polymerization, or serve as an anchoring group for the cationic polymerization of isobutylene, providing a potential access to novel materials with elastic properties and very low permeability to gases. See, e.g., De and White, Rubber Technologist's Handbook, Smithers Rapra Technology Limited: Shawbury, UK, 2011.
Industrial production of polyisoprene has mainly focused on anionic polymerization of the isoprene monomer, which controls the resulting polyisoprene double bond geometry to about a 5 to 1 (1,4-cis to 1,4-trans ratio). See, e.g., Matyjaszewski and Müller, Controlled and Living Polymerizations: From Mechanisms to Applications, Wiley-VCH, 2009. Catalysts based on rare earth metals such as neodymium afford ratios up to 50 to 1 (1,4-cis to 1,4-trans). See, e.g., Zhang et al., Angew. Chem. Int. Ed. (2007) 46:1909-1913; Zhang et al., Hou, Angew. Chem. Int. Ed. (2008) 47:2642-2645; Gao and Cui, J. Am. Chem. Soc. (2008) 130, 4984-4991; Ricci et al., Coord. Chem. Rev. (2010) 254:661-676; Nishiura and Hou, Nat. Chem. (2010) 2:257-268; and Li et al., Organometallics (2010) 29:2186. However, rare Earth metals are expensive and are only obtained by mining. Therefore, there is a need for a better, less expensive, way of making polymers from 1,3-dienes, such as polyisoprene, with a high selectivity, e.g., a selectivity for either 1,4-cis double bonds, 1,4-trans double bonds, 3,4-selectivity, or 1,2-selectivity, with less of an impact on the environment.